1. Field of the Invention
The invention relates to wavelength-selective optical filters that allow light of a narrow optical spectral band, centered around one wavelength, to pass through them while reflecting the wavelengths lying outside this band. Provision may be made for the central wavelength of the narrow spectral band to be adjusted by electrical means.
2. Background of the Invention
The word “light” is understood in the broad sense and in particular includes spectral bands in the infrared, as will be seen below, one main application of the invention being the filtering of light in the various fiber-optic telecommunications bands lying between 1.3 and 1.61 microns.
The advantage of these bands between 1.3 and 1.61 microns results from the fact that current optical fibers, made of glass, used in telecommunications networks have a low attenuation and the optical signals may therefore be transmitted over very great distances. In what follows, the invention will be explained with regard to this spectral band, it being understood that the invention can be transposed to other bands if the need to do so arises, using the materials suitable for these different bands. It is also understood that the invention is not limited to the telecommunications field, rather it may be employed in any field in which spectral analysis is required, such as for example in the petrochemical industry (as a hydrocarbon detector) or in the biological field (in blood analysis).
In a fiber-optic telecommunications network, a cable comprising several optical fibers may be used to produce several different transmission channels. It is also possible to carry out time-division multiplexing of the data in order to achieve the same result. However, the current trend, for a further increase in the data rate capacity of the network, is to transmit simultaneously, on the same optical fiber, several light wavelengths modulated independently of one another, each defining one data channel. The ITU (International Telecommunications Union) Standard 692 proposes to define adjacent channels with a 100 GHz optical spectral bandwidth that are centered on N adjacent normalized optical frequencies, the values of which are 200 terahertz, 199.9 terahertz, 199.8 terahertz, etc., corresponding to N wavelengths ranging from 1.52 microns up to 1.61 microns. In a channel with this bandwidth, it is possible to carry out light modulation at 10 to 40 gigabits per second without an excessive risk of interference with the channels of immediately adjacent spectral bands (using Gaussian modulation pulses to minimize the bandwidth occupied by this modulation). This frequency-division multiplexing technique is also called DWDM (Dense Wavelength Division Multiplexing).
In a telecommunications network, the problem is therefore to be able to collect the light corresponding to a given channel without disturbing the light in the neighboring channels. For example, at a transmission node of the network, assigned to transmitting data into channel i and for receiving data therefrom, it is necessary to be able to collect the light at a central frequency Fi (wavelength □i) without disturbing the transmission of the light modulating the central frequencies F1 to FN, although these optical frequencies are very close together.
To do this, there is a need to produce highly light-wavelength-selective optical filtering components capable of letting the central optical frequency Fi and the frequencies located within a narrow band of less than 50 GHz on either side of this frequency pass through them, while blocking the other bands. At the output of such a filter, only the light from channel i is collected and this can be demodulated in order to collect the useful data or to send it to another branch of the network.
More precisely, in order for it to be used in an optical telecommunications network, a filtering component must satisfy two major criteria:                a maximum modulation within a channel, which modulation must in practice be at most of the order of 0.5 dB. This modulation, well known in the literature as being called a “ripple”, is the maximum variation of the signal output by the filtering component over the spectral band of the channel in question; and        a minimum isolation between two adjacent channels, which in practice must be at least of the order of 20 dB. The isolation is defined as being the difference measured between the minimum amplitude of the signal output by the filtering component in the channel in question and the maximum amplitude in an adjacent channel.        
It has already been proposed to produce filtering components operating on the principle of Fabry-Perot interferometers produced by depositing semiconductor layers separated from each other by air gaps having thicknesses that are calibrated with respect to the wavelength □i to be selected. In practice, an interferometer comprises two mirrors consisting of superposed dielectric layers (Bragg mirrors), of high reflection coefficient, which are separated by a transparent plate of optical thickness k□i/2 (actual thickness k□i/2 if the plate is an air gap) where k is an integer defining the order of the interferometric filter. The mirrors, together with the space that separates them are called a cavity. Indiumphosphide (InP) is very suitable for these embodiments, in particular because of its transparency at the wavelengths in question, its very high refractive index, the possibility of growing layers of well-controlled thickness, and the possibility of using the technique of selective micromachining between InP layers and InGaAs layers.
If the layer thicknesses and the gaps between layers are very well controlled and if the materials have a large refractive index difference, such a filter proves to be highly selective with few layers or InP/air alternations.
Such a construction is described in the article by A. Spisser et al., entitled “Highly Selective 1.55 micrometer InP/airgap micromachined Fabry-Perot filter for optical communications” in Electronics Letters, No 34(5), pages 453-454, 1998. Other constructions have been proposed, made of micromachined silicon and of alloys based on gallium arsenide.
An intrinsic limitation occurs when a simple Fabry-Perot interferometer is used as a filtering component. Such a component does not make it possible to achieve, simultaneously, minimum ripple in a channel and sufficient isolation between two adjacent channels for use in an optical telecommunications network using a DWDM type multiplexing technique. This limitation will be better understood from FIG. 1 in which two mirrors a and b, having respective reflectivities Ra and Rb, define a Fabry-Perot cavity. The two mirrors a and b are separated from each other by a distance d. A light ray penetrates the filtering component at an angle of incidence θ. To simplify the reasoning, the mirrors a and b are considered to be infinite. In the particular case of a symmetrical cavity (Ra=Rb=R), the parameters λ and θ represent the wavelength and the angle of incidence, respectively, of the radiation in the cavity. The transmission curve T(λ) as a function of its wavelength λ is an Airy function and can be written as:
                                          T            1                    ⁡                      (            λ            )                          =                  1                      1            +                          M              ⁢                                                          ⁢                                                sin                  2                                ⁡                                  (                                                            2                      ⁢                      π                      ⁢                                                                                          ⁢                      n                      ⁢                                                                                          ⁢                      d                      ⁢                                                                                          ⁢                      cos                      ⁢                                                                                          ⁢                      θ                                        λ                                    )                                                                                        (        1        )            where
      M    =                  4        ⁢        R                              (                      1            -            R                    )                2              ,and where n is the optical index of the cavity medium.
We will now consider an air cavity so as not to encumber the notations. Of course, the invention is not limited to an air cavity, and any optical material of index n different from 1 may be used.
When the resonance condition is fulfilled, that is to say for a wavelength λp such that d cos θ=λp/2 (p being an integer representing the interference order), the transmission is a maximum and equal to 100%.
In the case of a Fabry-Perot cavity used as filter, the order may be kept fixed. This makes it possible to obtain a wavelength-tunability range bounded by the interval that separates two consecutive transmission peaks, this being called the free spectral interval (FSI). Tunability is achieved by varying the length d of the cavity.
To illustrate the limitations of a simple Fabry-Perot cavity, the following numerical example is chosen:                wavelength λ at normal incidence (θ=0): λ0=1550 nm;        interference order: p=4;        cavity length d0=2λ0=2×1550 nm;        R=99.6%.        
The aim is to obtain a filter for optical telecommunications with channels spaced apart by 100 GHz and a data rate of 10 Gb/s corresponding to a channel bandwidth of 0.2 nm (25 GHz) as described in the above paragraph.
The shape of the spectral response of the filter at normal incidence is shown in FIG. 2. It is centered on λ0=1550 nm. Being of Lorentzian type, it is quite different from the shape of an ideal filter, which would allow the entire bandwidth of the signal to pass through it and would cut off all of the rest. In this case, the ripple obtained is about 0.7 dB and the required −20 dB. The shape of the peak corresponding to an Airy function is therefore not satisfactory for the intended application.